A Russian military appeal court on Tuesday toughened a sentence for a left-wing academic who criticised Moscow's Ukraine offensive from a fine to five years in prison, his lawyer said.

The new sentence came two months after the prominent sociologist and Marxist thinker Boris Kagarlitsky was fined 600,000 rubles (around €6,100) for denigrating the Kremlin's military campaign on social media.

The fine was seen as a surprisingly lenient form of punishment for Kagarlitsky -- charged with "justifying terrorism" -- during Moscow's massive crackdown on dissent.

A Moscow region court on Tuesday "decided to change the sentence ... to five years' detention, instead of a fine," Kagarlitsky's lawyer Sergei Yerokhov said in a video on social media.

Kagarlitsky was arrested in July 2023 after he made a comment on social media alleging that a Ukrainian attack on Moscow's Crimea bridge was "more or less understandable".

His lawyer, Sergei Yerokhov, said on Tuesday that Kagarlitsky did not know his posts would be seen as illegal under Russian law and would have deleted them had he been told to do so.

He plans to appeal the verdict.

The 65-year-old is a well-known academic who has written extensively on Russian society and leftist political history. 

He taught at Moscow's Higher School of Economics -- once a university that was seen as a bastion of liberal thinking.  

Russia declared Kagarlitsky a "foreign agent" in 2022.